PROJECT SUMMARY/ABSTRACT Multiple systemic therapies are effective in the treatment of metastatic renal cell carcinoma (mRCC) and target several distinct molecular pathways, including vascular endothelial growth factor (VEGF) signaling, immune checkpoints, cytokine signaling, and the mechanistic target of rapamycin (mTOR) pathway. However, fewer than half of patients will respond to any one particular therapy, and the optimal sequencing of treatment remains unclear. Therefore, an area of urgent need in the care of patients with mRCC is the development of robust biomarkers that are predictive of treatment response. Small molecule inhibitors of VEGF receptors (VEGFR) have been the most preferred first-line treatment for mRCC patients for the past decade, and cabozantinib, a multi-kinase inhibitor of VEGFR, MET, and AXL was recently granted regulatory approval as frontline treatment. Preliminary studies from our group and others suggest that clear cell RCC (ccRCC) tumors characterized by high levels of expression of angiogenesis-associated genes and/or harboring somatic mutations in specific genes (i.e. PBRM1) may be more dependent on VEGF signaling and thus might better respond to VEGFR-targeted therapies. In addition, since tumor-associated angiogenesis may promote an immunosuppressive microenvironment and cabozantinib is known to have immuno-modulatory effects, we anticipate that the activation of specific immune pathways in the tumor microenvironment might be associated with response or resistance to this agent. In this application, we propose to assess candidate predictive biomarkers for response to cabozantinib by utilizing pre-treatment tumor specimens from patients with metastatic ccRCC (mccRCC) treated in the randomized phase III METEOR clinical trial that compared cabozantinib to the mTOR inhibitor everolimus. Specifically, we will test the hypothesis that expression of angiogenesis-associated genes is predictive of clinical response to cabozantinib (Aim 1). We will also assess tumor genetic alterations as predictive biomarker of clinical response to cabozantinib (Aim 2). Finally, we will explore gene signatures of immune pathways activation that might be associated with response or resistance to cabozantinib (Aim 3). The proposed prospective-retrospective study represents a unique opportunity for the development of clinically useful biomarkers that can significantly improve the treatment of patients with mRCC.